1. Filed of the Invention
The present invention relates to an impact drill for use in a drilling operation on the concrete, mortar or tile, for example, and more particularly to an impact drill having a drill mode for performing a drilling operation by rotating a drill bit and an impact drill mode for performing a drilling operation by rotating and vibrating the drill bit.
2. Description of the Related Art
FIG. 1 shows a conventional example of the impact drill of this kind. In FIG. 1, reference numeral 1 denotes a main frame portion that forms an outer shell of the impact drill and has the self-contained parts at predetermined positions, including a gear cover 17, an inner cover 18, an outer cover 19, a housing 7 and a handle portion 6. Reference numeral 2 denotes a spindle inserted transversely through the gear cover 17, and 3 denotes a drill chuck attached at the top end of the spindle. A rotational ratchet 4 is mounted near the central part of the spindle 2. The rotational ratchet 4 is rotated along with the rotation of the spindle 2, and moved along with the axial movement of the spindle 2. The serrated irregularities are formed on one face 4a of the rotational ratchet 4.
Reference numeral 5 denotes a stationary ratchet disposed at a position opposed to the rotational ratchet 4, in which the serrated irregularities are formed on one face 5a of the stationary ratchet. The stationary ratchet 5 has a hollow cylindrical shape, and is fixed to the inner cover 18, irrespective of the rotation and axial movement of the spindle 2.
On the other hand, a motor 8 is disposed inside the housing 7 linked to the handle portion 6. A rotational driving force of the motor 8 is transmitted via a gear 10 fixed to a rotation shaft 9 to a second pinion 11. The second pinion 11 has two pinion portions 11a, 11b having a different number of teeth, which are engaged with a low speed gear 12 and a high speed gear 13, respectively. When the second pinion 11 is rotated, both the gears 12, 13 are also rotated.
Reference numeral 14 denotes a clutch disk engaged with the spindle 2 and mounted to be slidable in the axial direction. If the clutch disk 14 is inserted into a concave portion of the low speed gear 12, the rotation of the second pinion 11 is transmitted via the low speed gear 12 and the clutch disk 14 to the spindle 2, as shown in FIG. 1. On the other hand, if the clutch disk 14 is slid to the right from the position of FIG. 1, and inserted into a concave portion of the high speed gear 13, the rotation of the second pinion 11 is transmitted via the high speed gear 13 and the clutch disk 14 to the spindle 2. Accordingly, the spindle 2 can be rotated at low speed or high speed by movement of the clutch disk 14.
Reference numeral 15 denotes a change lever for changing the operation mode of the impact drill, namely, between a drill mode and an impact drill mode. A change shaft 16 is press fit into the change lever 15, whereby when the change lever 15 is rotated, the change shaft 16 is also rotated. The change shaft 16 has a notch portion 16a, as shown in FIGS. 2, 3 and 4, whereby when the notch portion 16a is at the position of FIG. 2, the impact drill is operated in the drill mode, while when the notch portion 16ais at the position of FIG. 3, the impact drill is operated in the impact drill mode.
(A) Drill Mode
When a drill bit (not shown) attached in the drill chuck 3 is contacted with a machined surface and the handle portion 6 is pressed in a direction of the arrow in FIG. 1, an end part of the spindle 2 makes contact with the change shaft 16 to be immovable to the right, when the notch portion 16a of the change shaft 16 is at the position of FIG. 2. Accordingly, there is no contact between the irregular face 4a of the rotational ratchet 4 and the irregular face 5a of the stationary ratchet 5. Accordingly, a rotational driving force of the motor 8 is transmitted via the low speed gear 12 or high speed gear 13 to the spindle, so that the drill bit is given a rotational force.
(B) Impact Drill Mode
In an impact drill mode, the notch portion 16a of the change shaft 16 is brought into the position of FIG. 3 by rotating the change lever 15. Then, the drill bit attached in the drill chuck 3 is contacted with a machined surface. If the handle portion 6 is pushed in a direction of the arrow in FIG. 1, an end part of the spindle 2 enters the notch portion 16a, as shown in FIG. 4. That is, the spindle 2 is slightly moved to the right, so that the, irregular face 4a of the rotational ratchet 4 is contacted with the irregular face of the stationary ratchet 5.
In drilling the machined surface, if the spindle 2 is rotated in the state of FIG. 4, the rotational ratchet 4 is meshed and engaged with the stationary ratchet 5, and rotated to cause vibration due to the irregular faces of both the ratchets 4 and 5. This vibration is transmitted through the spindle 2 to the drill bit (not shown). That is, the drill bit is given a rotational force and vibration to perform a drilling operation.
However, when the impact drill described above is operated in the impact drill mode, the vibration caused by rotation of the spindle in the state where the irregular faces of the ratchets 4 and 5 are contacted under pressure is transmitted not only to the drill bit, but also through the stationary ratchet 5 and the inner cover 18 from the housing 7 to the handle portion 6. Therefore, there is a problem that the user of the impact drill undergoes a great vibration, and feels uncomfortable. Especially when the impact drill is continuously employed for a long time, care must be taken not to transmit the vibration to the user and cause adverse effect on the health of the user.
Several proposals for reducing the vibration transmitted to the user have been made. For example, in JP-B-2-30169, a structure was disclosed in which a clutch cam 22 is supported movably in the axial direction of the spindle 20, and biased and urged to a rotary cam 21 by a spring 23, as shown in FIG. 5.
In FIG. 5, reference numeral 21 denotes a rotary cam that is rotated along with the spindle 20. A cam face 21a of the rotary cam 21 is formed with serrated irregularities. On the other hand, the clutch cam 22 is composed of a hollow cylindrical portion slidable in the axial direction of the spindle 20 and a flange portion 22b. A cam face 22c of the flange portion 22b is formed with serrated irregularities.
The spring 23 is provided between the flange 22b of the clutch cam 22 and a plate 24a engaging a groove 22a of the clutch cam 22, and always urges the clutch cam 22 toward the rotary cam 21. Thus, when the spindle 20 is moved backward, the cam faces 21a and 22c are contacted under pressure. If a pressing force applied to the spindle 20 overcomes a resilience of the spring 23, the spring 23 is compressed, so that the clutch cam 22 is moved backward (to the right in the figure).
When the clutch cam 22 is moved forward from the back position due to a resilient force of the spring 23, it collides with the rotary cam 21, so that the rotary cam 21 is vibrated together with the spindle 20. With this structure, since the vibration caused by contact between the cam faces 21a and 22c is relieved by the spring 23 and transmitted to the handle portion (not shown), there is the effect that the vibration transmitted to the user is reduced as compared with the structure in which the ratchet 5 is firmly disposed as shown in FIG. 1.
In a case of the drill as disclosed in JP-B-2-30169, since the clutch cam 22 permits the spindle 20 to slide in the axial direction, and regulates the rotation, the slide faces 22e, 22e are vertically formed on both sides of the flange portion 22b, and the clutch cam 22 is carried between both the guide faces 26 of a retainer 24 extending from the plate 24a, as shown in FIG. 6.
When this structure has additionally a function of rotating the spindle 20 at high speed and low speed in the same manner as in FIG. 1, it has been found that there occurs a phenomenon that the impact force of the clutch cam 22 in colliding with the rotary cam 21 due to a restoring force of the spring 23 from the back position is weakened, as will be described later.